A process for methane fermentation treatment wherein an organic wastewater, an organic waste, or the like is treated by decomposition through methane fermentation is excellent in energy saving since energy for aeration is not required, excessive sludge is small, and energy can be recovered from a biogas generated as compared with aerobic treatment such as an active sludge process. However, since methane-producing bacteria or methane fermentation bacteria are low in growth rate and exhibit a bad precipitating ability, they are apt to flow out together with treated water. Therefore, it is difficult to increase microorganism density in a fermenter for use in methane fermentation treatment. Furthermore, there are problems in cost, building site, and the like.
As an anaerobic treatment process utilizing a highly efficient fermenter having a high microorganism density, there is an upflow anaerobic sludge blanket process (hereinafter referred to as “UASB”). It is a process recently widely used and has a characteristic that density of methane bacteria in a reactor can be maintained at a high density by granulation of anaerobic bacteria such as a methane bacterium in a granule form. As a result, an efficient treatment can be achieved even when the concentration of organic substances in the wastewater is considerably high.
However, in the conventional UASB process for a wastewater containing a high concentration of an inorganic sulfur compound such as hydrogen sulfide in a wastewater of chemical industries such as a wastewater of paper/pulp industries, since hydrogen sulfide in a raw water and hydrogen sulfide formed by sulfate reduction of sulfate ions inhibit methane fermentation, it is necessary to remove the same.
JP 5-84499 A discloses a methane fermentation treatment process wherein, in a medium-temperature methane fermentation of craft pulp wastewater by the upflow anaerobic sludge blanket process, sulfur matter in methanol-containing wastewater from a pulp cooking step was removed and then the wastewater was mixed with a wastewater containing polymeric hydrocarbons, followed by feeding the mixture to a methane fermentation reactor to treat it. However, there are problems that allowable sulfur matter is unclear and sulfur matter is excessively removed.
JP 2001-79590 A proposes a process of adding a hydrogen sulfide formation inhibitor to an organic wastewater containing a sulfate ion, in which hydrogen sulfide is generated, to form a sulfide and removing the formed sulfide through precipitation. In the process, a sulfide is formed by the action of a heavy metal contained in the hydrogen sulfide formation inhibitor and thereby hydrogen sulfide is precipitated and removed. In this process, however, there is a problem that methane fermentation is inhibited by the heavy metal contained therein.
Furthermore, JP 8-323387 A proposes a process for anaerobic treatment of an organic wastewater comprising adding an iron ion in an amount equimolar or more to the sulfur compound contained in the wastewater to the organic wastewater and conducting anaerobic treatment. In the process, there is a problem that iron is accumulated inside the methane fermenter since iron hydroxide flocks are formed from an iron ion excessively added relative to the sulfur compound.